If you live in the northern hemisphere, go out any night this week an hour or so after sunset and look at the western sky to catch a planetary triple play starring Venus, Saturn and Mars.

The first thing skywatchers will see — weather permitting — is the brilliant planet Venus, slightly north of west, in the constellation Gemini. Look for Gemini's twin first magnitude stars, Pollux and Castor, just above Venus.

As the sky gets darker, the planet Mars can be spotted to Venus' left as it appears in the constellation Leo very close to the bright, first magnitude star Regulus. Further still to the left will be Saturn shining in the western part of the constellation Virgo.

This sky map shows how to spot all three planets as they appear across a 71-degree angle in the night sky. For comparison, your closed fist held at arm's length covers about 5 degrees of arc in the sky.

Venus, Mars and Saturn are all currently appearing slightly north of the ecliptic, the path the sun appears to follow over the year, shown in green in the sky map. (More Mars photos.)

Note the positions of these three planets in relation to the bright background stars, because they are beginning an interesting journey which you will be able to follow over the next two months.

In early July, Venus will have moved rapidly to the left, crossing Cancer into Leo so that now it is next to the star Regulus. Mars, meanwhile, will have moved somewhat to the left. Saturn appears to have hardly moved at all.

By then, the three planets will now cover only 37 degrees in the sky, only half the spread they showed in early June.

A month after this, in the first week of August, the planets will be crowded into a 7-degree angle, and Mars will now be to the left of Saturn in Virgo. Venus, too, will have moved into Virgo.

All three will fit comfortably in the viewing field of a small pair of binoculars.

By August, Venus will still be brilliant, but both Saturn and Mars will have faded so that they just barely reach first magnitude. That's because Saturn and Mars are getting farther away from Earth, while Venus is getting closer.

From the southern hemisphere, the planets will appear in the same positions relative to each other, but the ecliptic will be almost vertical, and the planets arrayed one above the other, rather than forming an oblique angle with the horizon.

This will be a fine opportunity to observe the relative motion of three bright planets against a well marked background of stars, and to see the very different speeds at which they move: Venus traversing four constellations and Mars two, with Saturn hardly moving at all.